A rhythmic motor program, termed the scratch reflex, can be elicited by gentle mechanical stimulation of the shell in an immobilized, spinal turtle. Microelectrodes will be inserted into the somata of limb motor neurons in order to record the synaptic potentials controlling the activities of these cells. These recordings will reveal the output characteristics of the spinal cord generator for the scratch program. Manipulations of membrane voltage and cellular ionic content will allow us to explore the ionic mechanisms underlying these synaptic potentials. The descending neuronal signal driven by cutaneous afferents from the shell will be recorded and its effect on the scratch generator explored. Our hypothesis is that this decending signal may exhibit characteristics of "command" neurons found in invertebrates and fishes. The objectives of this work are to characterize the basic properties of turtle spinal cord neurons involved in the generation of motor programs. Since many features of spinal cord are invariant throughout the vertebrates, it is our hope that our results on turtle will serve as useful working hypotheses for the exploration of the motor mechanisms in more complex vertebrates.